The present invention relates to a cathode material of an electron beam device and a preparation method thereof, and more particularly, to a cathode material used as an electron emitting source of various evacuated electron beam devices and cathode-ray tubes, and a preparation method thereof.
Cathode-ray systems currently in widespread use are mostly based on emission systems with oxide cathode indirectly heated by a filament. However, since these systems have a limit in their emission ability, it is not possible to receive a current density of more than 1 A/cm2.
Also, the oxide cathode is fragile and adhesiveness to a metal substrate on which it is mounted is low, which results in early decay of cathode-ray systems with these types of cathodes. That is to say, in the case when even one of three oxide cathodes for a color picture tube is damaged, the whole system, which is expensive, will go out of order.
To overcome the drawback, attempts to apply highly efficient metal-alloy cathodes, which are free from the above-described problem, to a cathode-ray system have been actively made.
For example, it is known that a metal cathode based on lanthanum hexaboride (LaB6) is stronger than the oxide cathode and has a better emission capability than the oxide cathode. In fact, a monocrystal cathode of hexaboride can provide a high current density of about 10 A/cm2. However, despite the high emission ability of cathodes based on lanthanum hexaboride (LaB6), they have been used only in limited evacuated electron devices whose cathode units are replaceable since the lifetime of them is short. The short lifetime of cathodes based on lanthanum hexaboride (LaB6) is caused by their high activity to constituent materials of a heater. That is to say, lanthanum hexaboride (LaB6) contacts the heater constituent materials, for example, tungsten, to produce a number of fragile chemical compounds.
As a material having a high specific density of electron emission, alloy of iridium and small amounts of rare-earth metals of cerium group (lanthanum, cerium, praseodymium, neodymium, samarium) is well-known (see S. E. Rozhkov et al. xe2x80x9cWork Function of the Alloy of iridium with lanthanum, cerium, praseodymium, neodymium, samariumxe2x80x9d, Journal of Radiotechnika I electronika, 1969, Vol. 14, No. 5, p. 936-analogue).
However, the alloy is characterized by reduction in velocity of the drift of active components to the surface of the cathode which decreases during the operation of the cathode, resulting in a quick increase of the work function with time, decrease in the cathode""s emissive properties and decrease in the cathode""s resistance to ion bombardment. Also, it is not easy to manufacture a cathode unit using the binary alloy due to its brittleness. Further, since the melting point of the alloy is low, it may result in its collapse due to occasional and short-term technological leaps of temperature above the operating level. Thus, the above-described drawbacks of this material do not make it suitable for long-term and reliable operation of electronic devices.
In Author""s certificate of USSR No. 616662, published in Bulletin of Information No. 27, 1978-the analogue, a cathode material representing a ternary alloy of iridium, cerium and hafnium is disclosed. Although this material has excellent emission stability and high plasticity, it still has a low melting point, which makes it impossible to be applied to electron devices desired cathode of a high operating temperature.
Also, in Russian Federation Patent No. 2052855, a cathode material representing alloy of iridium, lanthanum or cerium, tungsten and/or rhenium is disclosed. According to this patent the lifetime of the cathode can be improved by tungsten or rhenium contained in the alloy. However, since tungsten or rhenium is brittle, it makes the cathode more brittle and thereby complicates the procedure of cathode manufacturing. It also decreases the electron-emissive properties of the cathode.
To solve the above problems, it is an objective of the present invention to provide a cathode material of an electron beam device with excellent electron-emissive property, prolonged lifetime and improved mechanical properties.
Accordingly, to achieve the above objective, there is provided a cathode material of an electron beam device comprising 0.5 to 9.0% by weight of a rare-earth metal of the cerium group, 0.5 to 15.0% by weight of tungsten and/or rhenium, 0.5 to 10% by weight of hafnium and the balance of iridium.
It is another objective of the present invention to provide a method of preparing a cathode material having excellent uniformity in view of its chemical composition and microstructure, without a residual gas.
According to another aspect of the present invention, there is provided a method of preparing a cathode material, including the steps of (a) preparing Ir5Ce by melting ridium and cerium, (b) preparing Hf3W by melting hafnium and tungsten, and (c) melting the alloy of the Ir5Ce and Hf3W prepared in steps (a) and (b) to prepare an ingot of the quaternary alloy.